An optical system for illuminating and viewing a target, which comprises a target, a source of illumination of the target and means for receiving the light remitted from the target, can be defined by an illumination axis and optical axis that converge at the target.
Such an optical system may be as simple as an operator of an illumination source viewing a target, wherein the operator embodies the means for receiving the light remitted from the target. An example of such an optical system is an operator of a vehicle, that is inside the vehicle and is looking out at an illuminated target such as a road or tunnel walls.
More complex optical systems include automated processors as means for receiving the light remitted from a viewed target. Examples of such optical systems can be found in diagnostic apparatuses such as endoscope devices. The endoscopes described in the art comprise an image pickup element and an illuminating element for illuminating an examined target.
For these optical systems it is advantageous to have the illuminating element and receiving means contained within a single compartment, namely behind a single optical window.
In a vehicle carrying an operator, the illuminating elements are usually situated outside the vehicle, thereby requiring the operator to leave the vehicle for repairs or the like. In vehicles such as submarines or trains travelling in a dark tunnel, this may be a perilous task.
In diagnostic apparatuses, especially those meant to be inserted into body orifices, having a single optical window is advisable for hygienic and practical considerations.
A frequent problem encountered in having the illumination element and means for receiving remitted light contained behind a single optical window is the “noise” (backscatter and stray light) produced by light remitted from the optical window itself, which is received by the receiving means.
Presently used techniques for reducing noise include utilizing light guiding means, or separating the illumination element from the receiving means.
For example, U.S. Pat. No. 5,840,014 (Miyano et al.) describes an endoscope having an illumination window and a viewing window having a detachable protective covering and a transparent material for purging air from the space between the front end and the detachable covering, for lowering loss in illumination light quantity.